Expression of Pseudomonas syringae type III effectors in yeast under stress conditions reveals that HopX1 attenuates activation of the high osmolarity glycerol MAP kinase pathway

Unravelling bacterial virulence factors in yeast: From identification to the elucidation of their mechanisms of action

Pathogenic bacteria employ virulence factors (VF) to establish infection and cause disease in their host. Yeasts, Saccharomyces cerevisiae and Saccharomyces pombe, are useful model organisms to study the functions of bacterial VFs and their interaction with targeted cellular processes because yeast processes and organelle structures are highly conserved and similar to higher eukaryotes. In this review, we describe the principles and applications of the yeast model for the identification and functional characterisation of bacterial VFs to investigate bacterial pathogenesis. The growth inhibition phenotype caused by the heterologous expression of bacterial VFs in yeast is commonly used to identify candidate VFs. Then, subcellular localisation patterns of bacterial VFs can provide further clues about their target molecules and functions during infection. Yeast knockout and overexpression libraries are also used to investigate VF interactions with conserved eukaryotic cell structures (e.g., cytoskeleton and plasma membrane), and cellular processes (e.g., vesicle trafficking, signalling pathways, and programmed cell death). In addition, the yeast growth inhibition phenotype is also useful for screening new drug leads that target and inhibit bacterial VFs. This review provides an updated overview of new tools, principles and applications to study bacterial VFs in yeast.

FlgI Is a Sec-Dependent Effector of Candidatus Liberibacter asiaticus That Can Be Blocked by Small Molecules Identified Using a Yeast Screen

Huanglongbing (HLB) is one of the most devastating diseases of citrus worldwide. The phloem-restricted bacterium Candidatus Liberibacter asiaticus (CLas) is considered to be the main pathogen responsible for HLB. There is currently no effective practical strategy for the control of HLB. Our understanding of how pathogens cause HLB is limited because CLas has not been artificially cultured. In this study, 15 potential virulence factors were predicted from the proteome of CLas through DeepVF and PHI-base searches. One among them, FlgI, was found to inhibit yeast growth when expressed in Saccharomyces cerevisiae. The expression of the signal peptide of FlgI fused with PhoA in Escherichia coli resulted in the discovery that FlgI was a novel Sec-dependent secretory protein. We further found that the carboxyl-terminal HA-tagged FlgI was secreted via outer membrane vesicles in Sinorhizobium meliloti. Fluoresence localization of transient expression FlgI-GFP in Nicotiana benthamiana revealed that FlgI is mainly localized in the cytoplasm, cell periphery, and nuclear periphery of tobacco cells. In addition, our experimental results suggest that FlgI has a strong ability to induce callose deposition and cell necrosis in N. benthamiana. Finally, by screening a large library of compounds in a high-throughput format, we found that cyclosporin A restored the growth of FlgI-expressing yeast. These results confirm that FlgI is a novel Sec-dependent effector, enriching our understanding of CLas pathogenicity and helping to develop new and more effective strategies to manage HLB.

Natural variation in a short region of the Acidovorax citrulli type III-secreted effector AopW1 is associated with differences in cytotoxicity and host adaptation

Bacterial fruit blotch, caused by Acidovorax citrulli, is a serious disease of melon and watermelon. The strains of the pathogen belong to two major genetic groups: group I strains are strongly associated with melon, while group II strains are more aggressive on watermelon. A. citrulli secretes many protein effectors to the host cell via the type III secretion system. Here we characterized AopW1, an effector that shares similarity to the actin cytoskeleton-disrupting effector HopW1 of Pseudomonas syringae and with effectors from other plant-pathogenic bacterial species. AopW1 has a highly variable region (HVR) within amino acid positions 147 to 192, showing 14 amino acid differences between group I and II variants. We show that group I AopW1 is more toxic to yeast and Nicotiana benthamiana cells than group II AopW1, having stronger actin filament disruption activity, and increased ability to induce cell death and reduce callose deposition. We further demonstrated the importance of some amino acid positions within the HVR for AopW1 cytotoxicity. Cellular analyses revealed that AopW1 also localizes to the endoplasmic reticulum, chloroplasts, and plant endosomes. We also show that overexpression of the endosome-associated protein EHD1 attenuates AopW1-induced cell death and increases defense responses. Finally, we show that sequence variation in AopW1 plays a significant role in the adaptation of group I and II strains to their preferred hosts, melon and watermelon, respectively. This study provides new insights into the HopW1 family of bacterial effectors and provides first evidence on the involvement of EHD1 in response to biotic stress.

The Antibacterial and Anti-Eukaryotic Type VI Secretion System MIX-Effector Repertoire in Vibrionaceae

Vibrionaceae is a widespread family of aquatic bacteria that includes emerging pathogens and symbionts. Many Vibrionaceae harbor a type VI secretion system (T6SS), which is a secretion apparatus used to deliver toxins, termed effectors, into neighboring cells. T6SSs mediate both antibacterial and anti-eukaryotic activities. Notably, antibacterial effectors are encoded together with a gene that encodes a cognate immunity protein so as to antagonize the toxicity of the effector. The MIX (Marker for type sIX effectors) domain has been previously defined as a marker of T6SS effectors carrying polymorphic C-terminal toxins. Here, we set out to identify the Vibrionaceae MIX-effector repertoire and to analyze the various toxin domains they carry. We used a computational approach to search for the MIX-effectors in the Vibrionaceae genomes, and grouped them into clusters based on the C-terminal toxin domains. We classified MIX-effectors as either antibacterial or anti-eukaryotic, based on the presence or absence of adjacent putative immunity genes, respectively. Antibacterial MIX-effectors carrying pore-forming, phospholipase, nuclease, peptidoglycan hydrolase, and protease activities were found. Furthermore, we uncovered novel virulence MIX-effectors. These are encoded by “professional MIXologist” strains that employ a cocktail of antibacterial and anti-eukaryotic MIX-effectors. Our findings suggest that certain Vibrionaceae adapted their antibacterial T6SS to mediate interactions with eukaryotic hosts or predators.

Expression of HopAI interferes with MAP kinase signalling in Magnaporthe oryzae

The Pmk1 and Mps1 MAP kinases are essential for appressorium formation and plant infection in Magnaporthe oryzae. However, their exact roles during invasive growth are not clear because pmk1 and mps1 mutants are defective in penetration. To further characterize their functions after penetration, in this study we expressed the Pseudomonas syringae effector HopAI known to inactivate plant MAP kinases in M. oryzae. Constitutive expression of HopAI with the RP27 or TrpC promoter resulted in defects in hyphal growth, conidiation, appressorium penetration and pathogenicity, which is similar to the phenotype of the mps1 mutant. HopAI interacted strongly with Mps1 in vivo and expression of dominant active MKK2 partially suppressed the defects of P_RP27 -HopAI transformants, which were significantly reduced in Mps1 phosphorylation. When the infection-specific MIR1 (Magnaporthe-infection-related gene-1) promoter was used to express HopAI, P_MIR1 -HopAI transformants were defective in the spreading of invasive hyphae and elicited strong defense responses in penetrated plant cells. Expression of HopAI in Fusarium graminearum also mainly affected the activation of Mgv1, an Mps1 orthologue. Taken together, our results showed that Mps1 is the major intracellular target of HopAI when it is overexpressed, and MAP kinase signalling is important for cell-to-cell movement of invasive hyphae in M. oryzae.

VopE, a Vibrio cholerae Type III Effector, Attenuates the Activation of CWI-MAPK Pathway in Yeast Model System

VopE, a mitochondrial targeting T3SS effector protein of Vibrio cholerae, perturbs innate immunity by modulating mitochondrial dynamics. In the current study, ectopic expression of VopE was found to be toxic in a yeast model system and toxicity was further aggravated in the presence of various stressors. Interestingly, a VopE variant lacking predicted mitochondrial targeting sequence (MTS) also exhibited partial lethality in the yeast system. With the aid of yeast genetic tools and different stressors, we have demonstrated that VopE and its derivative VopE^ΔMTS modulate cell wall integrity (CWI-MAPK) signaling pathway and have identified several critical residues contributing to the lethality of VopE. Furthermore, co-expression of two effectors VopE^ΔMTS and VopX, interfering with the CWI-MAPK cellular pathway can partially suppress the VopX mediated yeast growth inhibition. Taken together, these results suggest that VopE alters signaling through the CWI-MAPK pathway, and demonstrates the usefulness of yeast model system to gain additional insights on the functionality of VopE.

The effector AWR5 from the plant pathogen Ralstonia solanacearum is an inhibitor of the TOR signalling pathway

Bacterial pathogens possess complex type III effector (T3E) repertoires that are translocated inside the host cells to cause disease. However, only a minor proportion of these effectors have been assigned a function. Here, we show that the T3E AWR5 from the phytopathogen Ralstonia solanacearum is an inhibitor of TOR, a central regulator in eukaryotes that controls the switch between cell growth and stress responses in response to nutrient availability. Heterologous expression of AWR5 in yeast caused growth inhibition and autophagy induction coupled to massive transcriptomic changes, unmistakably reminiscent of TOR inhibition by rapamycin or nitrogen starvation. Detailed genetic analysis of these phenotypes in yeast, including suppression of AWR5-induced toxicity by mutation of CDC55 and TPD3, encoding regulatory subunits of the PP2A phosphatase, indicated that AWR5 might exert its function by directly or indirectly inhibiting the TOR pathway upstream PP2A. We present evidence in planta that this T3E caused a decrease in TOR-regulated plant nitrate reductase activity and also that normal levels of TOR and the Cdc55 homologues in plants are required for R. solanacearum virulence. Our results suggest that the TOR pathway is a bona fide T3E target and further prove that yeast is a useful platform for T3E function characterisation.

Direct and Indirect Targeting of PP2A by Conserved Bacterial Type-III Effector Proteins

Bacterial AvrE-family Type-III effector proteins (T3Es) contribute significantly to the virulence of plant-pathogenic species of Pseudomonas, Pantoea, Ralstonia, Erwinia, Dickeya and Pectobacterium, with hosts ranging from monocots to dicots. However, the mode of action of AvrE-family T3Es remains enigmatic, due in large part to their toxicity when expressed in plant or yeast cells. To search for targets of WtsE, an AvrE-family T3E from the maize pathogen Pantoea stewartii subsp. stewartii, we employed a yeast-two-hybrid screen with non-lethal fragments of WtsE and a synthetic genetic array with full-length WtsE. Together these screens indicate that WtsE targets maize protein phosphatase 2A (PP2A) heterotrimeric enzyme complexes via direct interaction with B' regulatory subunits. AvrE1, another AvrE-family T3E from Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000), associates with specific PP2A B' subunit proteins from its susceptible host Arabidopsis that are homologous to the maize B' subunits shown to interact with WtsE. Additionally, AvrE1 was observed to associate with the WtsE-interacting maize proteins, indicating that PP2A B' subunits are likely conserved targets of AvrE-family T3Es. Notably, the ability of AvrE1 to promote bacterial growth and/or suppress callose deposition was compromised in Arabidopsis plants with mutations of PP2A genes. Also, chemical inhibition of PP2A activity blocked the virulence activity of both WtsE and AvrE1 in planta. The function of HopM1, a Pto DC3000 T3E that is functionally redundant to AvrE1, was also impaired in specific PP2A mutant lines, although no direct interaction with B' subunits was observed. These results indicate that sub-component specific PP2A complexes are targeted by bacterial T3Es, including direct targeting by members of the widely conserved AvrE-family.

Yeast as a Heterologous Model System to Uncover Type III Effector Function

Type III effectors (T3E) are key virulence proteins that are injected by bacterial pathogens inside the cells of their host to subvert cellular processes and contribute to disease. The budding yeast Saccharomyces cerevisiae represents an important heterologous system for the functional characterisation of T3E proteins in a eukaryotic environment. Importantly, yeast contains eukaryotic processes with low redundancy and are devoid of immunity mechanisms that counteract T3Es and mask their function. Expression in yeast of effectors from both plant and animal pathogens that perturb conserved cellular processes often resulted in robust phenotypes that were exploited to elucidate effector functions, biochemical properties, and host targets. The genetic tractability of yeast and its amenability for high-throughput functional studies contributed to the success of this system that, in recent years, has been used to study over 100 effectors. Here, we provide a critical view on this body of work and describe advantages and limitations inherent to the use of yeast in T3E research. “Favourite” targets of T3Es in yeast are cytoskeleton components and small GTPases of the Rho family. We describe how mitogen-activated protein kinase (MAPK) signalling, vesicle trafficking, membrane structures, and programmed cell death are also often altered by T3Es in yeast and how this reflects their function in the natural host. We describe how effector structure–function studies and analysis of candidate targeted processes or pathways can be carried out in yeast. We critically analyse technologies that have been used in yeast to assign biochemical functions to T3Es, including transcriptomics and proteomics, as well as suppressor, gain-of-function, or synthetic lethality screens. We also describe how yeast can be used to select for molecules that block T3E function in search of new antibacterial drugs with medical applications. Finally, we provide our opinion on the limitations of S. cerevisiae as a model system and its most promising future applications.

Identification of Critical Amino Acids Conferring Lethality in VopK, a Type III Effector Protein of Vibrio cholerae: Lessons from Yeast Model System

VopK, a type III effector protein, has been implicated in the pathogenesis of Vibrio cholerae strains belonging to diverse serogroups. Ectopic expression of this protein exhibits strong toxicity in yeast model system. In order to map critical residues in VopK, we scanned the primary sequence guided by available data on various toxins and effector proteins. Our in silico analysis of VopK indicated the presence of predicted MCF1-SHE (SHxxxE) serine peptidase domain at the C-terminus region of the protein. Substitution of each of the predicted catalytic triad residues namely Ser³¹⁴, His³⁵³ and Glu³⁵⁷ with alanine resulted in recombinant VopK proteins varying in lethality as evaluated in yeast model system. We observed that replacement of glutamate³⁵⁷ to alanine causes complete loss in toxicity while substitutions of serine³¹⁴ and histidine³⁵³ with alanine exhibited partial loss in toxicity without affecting the stability of variants. In addition, replacement of another conserved serine residue at position 354 (S³⁵⁴) within predicted S³¹⁴H³⁵³E³⁵⁷ did not affect toxicity of VopK. In essence, combined in silico and site directed mutagenesis, we have identified critical amino acids contributing to the lethal activity of VopK in yeast model system.

Novel type III effectors in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen that causes chronic and acute infections in immunocompromised patients. Most P. aeruginosa strains encode an active type III secretion system (T3SS), utilized by the bacteria to deliver effector proteins from the bacterial cell directly into the cytoplasm of the host cell. Four T3SS effectors have been discovered and extensively studied in P. aeruginosa: ExoT, ExoS, ExoU, and ExoY. This is especially intriguing in light of P. aeruginosa’s ability to infect a wide range of hosts. We therefore hypothesized that additional T3SS effectors that have not yet been discovered are encoded in the genome of P. aeruginosa. Here, we applied a machine learning classification algorithm to identify novel P. aeruginosa effectors. In this approach, various types of data are integrated to differentiate effectors from the rest of the open reading frames of the bacterial genome. Due to the lack of a sufficient learning set of positive effectors, our machine learning algorithm integrated genomic information from another Pseudomonas species and utilized dozens of features accounting for various aspects of the effector coding genes and their products. Twelve top-ranking predictions were experimentally tested for T3SS-specific translocation, leading to the discovery of two novel T3SS effectors. We demonstrate that these effectors are not part of the injection structural complex and report initial efforts toward their characterization.

Pseudomonas aeruginosa uses a type III secretion system (T3SS) to secrete toxic proteins, termed effectors, directly into the cytoplasm of the host cell. The activation of this secretion system is correlated with disease severity and patient death. Compared with many other T3SS-utilizing pathogenic bacteria, P. aeruginosa has a fairly limited arsenal of effectors that have been identified. This is in sharp contrast with the wide range of hosts that this bacterium can infect. The discovery of two novel effectors described here is an important step toward better understanding of the virulence and host evasion mechanisms adopted by this versatile pathogen and may provide novel approaches to treat P. aeruginosa infections.

Vibrio type III effector VPA1380 is related to the cysteine protease domain of large bacterial toxins

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium and one of the leading causes of food-borne gastroenteritis. Its genome harbors two Type III Secretion Systems (T3SS1 and T3SS2), but only T3SS2 is required for enterotoxicity seen in animal models. Effector proteins secreted from T3SS2 have been previously shown to promote colonization of the intestinal epithelium, invasion of host cells, and destruction of the epithelial monolayer. In this study, we identify VPA1380, a T3SS2 effector protein that is toxic when expressed in yeast. Bioinformatic analyses revealed that VPA1380 is highly similar to the inositol hexakisphosphate (IP6)-inducible cysteine protease domains of several large bacterial toxins. Mutations in conserved catalytic residues and residues in the putative IP6-binding pocket abolished toxicity in yeast. Furthermore, VPA1380 was not toxic in IP6 deficient yeast cells. Therefore, our findings suggest that VPA1380 is a cysteine protease that requires IP6 as an activator.

Effectors of animal and plant pathogens use a common domain to bind host phosphoinositides

Bacterial Type III Secretion Systems deliver effectors into host cells to manipulate cellular processes to the advantage of the pathogen. Many host targets of these effectors are found on membranes. Therefore, to identify their targets, effectors often use specialized membrane-localization domains to localize to appropriate host membranes. However, the molecular mechanisms used by many domains are unknown. Here we identify a conserved bacterial phosphoinositide-binding domain (BPD) that is found in functionally diverse Type III effectors of both plant and animal pathogens. We show that members of the BPD family functionally bind phosphoinositides and mediate localization to host membranes. Moreover, NMR studies reveal that the BPD of the newly identified Vibrio parahaemolyticus Type III effector VopR is unfolded in solution, but folds into a specific structure upon binding its ligand phosphatidylinositol-(4,5)-bisphosphate. Thus, our findings suggest a possible mechanism for promoting refolding of Type III effectors after delivery into host cells.

Identification of novel Coxiella burnetii Icm/Dot effectors and genetic analysis of their involvement in modulating a mitogen-activated protein kinase pathway

Coxiella burnetii, the causative agent of Q fever, is a human intracellular pathogen that utilizes the Icm/Dot type IVB secretion system to translocate effector proteins into host cells. To identify novel C. burnetii effectors, we applied a machine-learning approach to predict C. burnetii effectors, and examination of 20 such proteins resulted in the identification of 13 novel effectors. To determine whether these effectors, as well as several previously identified effectors, modulate conserved eukaryotic pathways, they were expressed in Saccharomyces cerevisiae. The effects on yeast growth were examined under regular growth conditions and in the presence of caffeine, a known modulator of the yeast cell wall integrity (CWI) mitogen-activated protein (MAP) kinase pathway. In the presence of caffeine, expression of the effectors CBU0885 and CBU1676 caused an enhanced inhibition of yeast growth, and the growth inhibition of CBU0388 was suppressed. Furthermore, analysis of synthetic lethality effects and examination of the activity of the CWI MAP kinase transcription factor Rlm1 indicated that CBU0388 enhances the activation of this MAP kinase pathway in yeast, while CBU0885 and CBU1676 abolish this activation. Additionally, coexpression of CBU1676 and CBU0388 resulted in mutual suppression of their inhibition of yeast growth. These results strongly indicate that these three effectors modulate the CWI MAP kinase pathway in yeast. Moreover, both CBU1676 and CBU0885 were found to contain a conserved haloacid dehalogenase (HAD) domain, which was found to be required for their activity. Collectively, our results demonstrate that MAP kinase pathways are most likely targeted by C. burnetii Icm/Dot effectors.

Genomics-Based Exploration of Virulence Determinants and Host-Specific Adaptations of Pseudomonas syringae Strains Isolated from Grasses

The Pseudomonas syringae species complex has recently been named the number one plant pathogen, due to its economic and environmental impacts, as well as for its role in scientific research. The bacterium has been repeatedly reported to cause outbreaks on bean, cucumber, stone fruit, kiwi and olive tree, as well as on other crop and non-crop plants. It also serves as a model organism for research on the Type III secretion system (T3SS) and plant-pathogen interactions. While most of the current work on this pathogen is either carried out on one of three model strains found on dicot plants with completely sequenced genomes or on isolates obtained from recent outbreaks, not much is known about strains isolated from grasses (Poaceae). Here, we use comparative genomics in order to identify putative virulence-associated genes and other Poaceae-specific adaptations in several newly available genome sequences of strains isolated from grass species. All strains possess only a small number of known Type III effectors, therefore pointing to the importance of non-Type III secreted virulence factors. The implications of this finding are discussed.

Synthetic biology: lessons from engineering yeast MAPK signalling pathways

All living cells respond to external stimuli and execute specific physiological responses through signal transduction pathways. Understanding the mechanisms controlling signalling pathways is important for diagnosing and treating diseases and for reprogramming cells with desired functions. Although many of the signalling components in the budding yeast Saccharomyces cerevisiae have been identified by genetic studies, many features concerning the dynamic control of pathway activity, cross-talk, cell-to-cell variability or robustness against perturbation are still incompletely understood. Comparing the behaviour of engineered and natural signalling pathways offers insight complementary to that achievable with standard genetic and molecular studies. Here, we review studies that aim at a deeper understanding of signalling design principles and generation of novel signalling properties by engineering the yeast mitogen-activated protein kinase (MAPK) pathways. The underlying approaches can be applied to other organisms including mammalian cells and offer opportunities for building synthetic pathways and functionalities useful in medicine and biotechnology.